sparc64: refactor code in viohs.c

The sparc64 allmodconfig build broke due to enabling of the
branch_tracer that does some very clever things with
all if conditions. This caused my gcc 3.4.5 to be so confused that
it emitted a warning:

arch/sparc/kernel/viohs.c: In function `vio_control_pkt_engine':
arch/sparc/kernel/viohs.c:335: warning: 'nver' might be used uninitialized in this function

And with -Werror this broke the build.

Refactor code so it:
1) becomes more readable
2) no longer emit a warning with the branch_tracer enabled

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>